Automobiles and trucks including pickup trucks, known hereafter as tow vehicles, have long been used to tow trailers. Typically, the tow vehicle has a ball and socket coupling mounted on the rear of the tow vehicle. The ball which may be known as a hitch ball projects upwards in the vicinity of the rear bumper of the tow vehicle. The trailer typically has a tongue which has a socket which fits over and closes around the hitch ball to secure the trailer to the tow vehicle so that the trailer may turn relative to this tow vehicle by the socket rotating with respect to the hitch ball. Furthermore, the trailer can rock forward or rock rearward with respect to the tow vehicle and rock right and left with respect to the tow vehicle for forming a universal pivotal movement.
The offset of the hitch from the rear axle of the tow vehicle reduces the overall stability of the combination of trailer and tow vehicle. At the higher tow vehicle speeds, this loss in stability is increased, and at these higher speeds, wind induced forces against the trailer can cause a swaying motion known as fish tailing. Similar to the effect of wind, air turbulence can cause fish tailing and can be gentle or severe; this air turbulence may be generated by large other vehicles traveling at high rates of speed. Some vehicles displace a large amount of air and create turbulence that affects all vehicles which happen to be near it. Inertial forces created by uneven road surfaces may also induce fish tailing. If one wheel of the trailer or tow vehicle is riding on the shoulder, when the wheel returns to the pavement, the trailer will be subject to a sudden jolt which may induce fish tailing.
Regardless of a source of the fish tailing, fish tailing is dangerous because it may cause the driver of the tow vehicle to lose control of both the tow vehicle and the trailer. Additionally, fish tailing induces severe stresses on the trailer hitch and frame components of both the tow vehicle and the trailer and may cause damage or failure. Fish tailing imposes severe demands on the suspension system of the tow vehicle and may cause components such as shock absorbers, bushings and springs excessive wear or even failure.
From the above discussion, it would seem logical that large over the road tractor-trailer vehicles would because of their large size be more susceptible to fish tailing; however, because the pivot axes for the trailer is substantially over the real axle of the tractor, the tractor-trailer vehicles are generally more stable. The hitch for the tractor-trailer is located near the rear axle of the tractor, and consequently these tractor-trailers exhibit stability at high speeds.
There are currently devices available for reducing swaying and fish tailing; however, these devices tend to stiffen the pivoting ability of the hitch and socket. Additionally, these devices are not very effective and result in stress being applied to the tow vehicle and to trailer to which they are attached. This stress is especially strong while the tow vehicle and trailer are turning sharply while parking. Attempts have been made to stabilize trailers by employing converging links in the hitch mechanism and these links have the effect of displacing the effective pivot axes for the hitch mechanism in the forward direction despite the fact that the hitch is not located at the position where the axis is displaced. One such link arrangement is shown in U.S. Pat. No. 4,019,754 to Hinckley. These types of link arrangements cannot accommodate 90 degree turns and consequently are of limited value in urban areas or areas of confined parking. Another patent which deals with the above-mentioned subject matter is U.S. Pat. No. 4,722,542 to Hensley. Hensley discloses a complicated hitch assembly having a set of complicated parts which add cost. This hitch assembly requires properly tuned electric brakes on trailers and produces undesirable tracking in sharp turns.